The present invention relates in general to optical waveguide devices, and in particular to an optical waveguide having a vertically-tapered waveguide core which can be used to expand an optical mode of light therein in the vertical direction and also to form an optical spot-size transformer that is useful for coupling light between the optical waveguide and a single-mode optical fiber.
Optical waveguides formed on a substrate are planar devices in which an optical mode of light therein can be highly elliptical. In order to efficiently couple light between the optical waveguide and a single-mode optical fiber, expansion of the optical mode in the vertical direction (i.e. normal to the plane of the substrate) and/or the horizontal direction (i.e. parallel to the plane of the substrate) is generally required. Different approaches have been proposed for doing this depending upon whether the optical waveguide is formed from epitaxial semiconductor materials or dielectric materials.
A horizontal mode expansion is generally relatively simple to effect since this can be done by photolithographic patterning of one or more layers of the optical waveguide structure (see e.g. U.S. Pat. No. 6,229,947 which is incorporated herein by reference). On the other hand, a vertical mode expansion is relatively difficult since this requires a change in thickness of one or more layers of the optical waveguide over distance.
Various approaches for forming an optical waveguide providing a vertical mode expansion have been reported so far, but these are not straightforward. These approaches, which have been tried with limited success, include the use of special epitaxial growth or etching techniques (see I. Moerman et al, xe2x80x9cA Review on Fabrication Technologies for the Monolithic Integration of Tapers with III-V Semiconductor Devices,xe2x80x9d IEEE Journal of Selected Topics in Quantum Electronics, vol. 3, pp. 1308-1320, December 1997; A. Chen et al, xe2x80x9cVertically Tapered Polymer Waveguide Mode Size Transformer for Improved Fiber Coupling,xe2x80x9d Optical Engineering, vol. 39, pp. 1507-1516, June 2000); selective fluorination of a polyimide by electron beam irradiation (see R. Inaba et al, xe2x80x9cTwo-Dimensional Mode Size Transformation by xcex94n-Controlled Polymer Waveguides,xe2x80x9d IEEE Journal of Lightwave Technology, vol. 16, pp. 620-624, April 1998); forming the optical waveguide by deposition of polymers in a vertically-tapered trench etched below the surface of a substrate (T. Bakke et al, xe2x80x9cPolymeric Optical Mode Converter for Hybrid Photonic Integrated Circuits,xe2x80x9d Proceedings of the SPIE Conference on Optoelectronic Interconnects VI, pp. 234-241, January 1999); and the use of multiple planar waveguiding layers (R. S. Fan et al, xe2x80x9cTapered Polymer Single-Mode Waveguides for Mode Transformation,xe2x80x9d IEEE Journal of Lightwave Technology, vol. 17, pp. 466-474, March 1999).
The present invention represents an advance over the prior art by providing a relatively simple and inexpensive way of forming a vertically-tapered optical waveguide section by varying the thickness of a waveguide core layer in response to the width of an underlying mesa structure that can be photolithographically defined and patterned with a fixed step height. The vertically-tapered optical waveguide section can be incorporated into the optical waveguide together with a laterally-tapered optical waveguide section to form an optical spot-size transformer for efficiently coupling light between the optical waveguide and a single-mode optical fiber.
The present invention relates to an optical waveguide formed on a substrate and comprising a waveguide core sandwiched between an upper cladding layer and a lower cladding layer, with the lower cladding layer being patterned to form a mesa structure having a width that varies with distance along at least a portion of the length of the optical waveguide, and with the thickness of the waveguide core varying in proportion to the width of the mesa structure. The waveguide core comprises a spin-coatable material which can be a polymer, a sol gel, or a spin-on glass. The upper and lower cladding layers also comprise spin-coatable materials which have indices of refraction that are different (e.g. smaller) from the index of refraction of the waveguide core.
The width of the mesa structure increases nonlinearly with distance over the portion of the mesa structure wherein the width varies. The remainder of the mesa structure can be fixed (i.e. constant) in width. The nonlinear variation in width of the mesa structure can, in some instances, result in a substantially linear variation in thickness of the overlying waveguide core, for example, when the variation in the width of the mesa structure is exponential. The upper cladding layer can be patterned to provide a uniform width over a major part of the length thereof, and in some instances (e.g. to form an optical spot-size transformer using the optical waveguide) can further be patterned to provide a variable (e.g. tapered) width over a minor part of the length thereof. For the part of the upper cladding layer wherein the width is variable, the underlying waveguide core generally has a fixed layer thickness.
The optical waveguide of the present invention is compatible with many different types of substrate materials including semiconductors, glasses, fused silica, sapphire, metals or metal alloys, ceramics, polymers, resins and printed wiring boards. When the substrate comprises silicon, the substrate can further include an insulating layer (e.g. a thermal oxide, a low-pressure chemical-vapor-deposited. material, or a plasma-enhanced chemical-vapor-deposited material) formed on an upper side of the substrate below the cladding layer. To aid in forming the optical waveguide, a first silicon oxynitride etch-stop layer can be deposited over the mesa structure; and a second silicon oxynitride etch-stop layer can be deposited over the waveguide core.
For use in the visible and near-infrared wavelength range, the waveguide core generally has a thickness in the range of 0.2-4 microns (xcexcm). The upper cladding layer can have a width, for example, in the range of 1 xcexcm to one centimeter. The mesa structure formed in the lower cladding layer can have a width, for example, in the range of 5-250 xcexcm, with the portion of the mesa structure wherein the width varies with distance being, for example, about 100-1000 xcexcm long.
The present invention also relates to an optical spot-size transformer for coupling light between an optical fiber and an optical waveguide formed on a substrate, with the optical spot-size transformer including a first section of the optical waveguide located proximate to the optical fiber for altering a lateral dimension of an optical mode of the light, and a second section of the optical waveguide located distal to the optical fiber for altering a vertical dimension of the optical mode of the light. Each section of the optical waveguide comprises a waveguide core sandwiched between an upper cladding layer and a mesa structure formed in a lower cladding layer. In the first section, the waveguide core and the mesa structure are both substantially uniform in width and height, and the upper cladding layer has a substantially uniform height and a nonuniform width that increases with distance away from the optical fiber. In the second section, the height of the waveguide core and the width of the mesa structure both increase with distance away from the optical fiber.
The optical waveguide can be formed as described above, with the waveguide core comprising a spin-coatable material such as a polymer, sol gel, or spin-on glass. The optical waveguide can be formed on a supporting substrate as described previously. To provide coupling of light to or from a single-mode optical fiber, the upper cladding layer can be laterally tapered to provide a minimum width of 0.5-2 microns at an end of the upper cladding layer that faces the optical fiber. The width of the mesa structure in the second section of the optical waveguide preferably increases nonlinearly with the distance away from the optical fiber.
The present invention is further related to a method for forming a vertically-tapered optical waveguide on a substrate, comprising steps for forming a mesa structure on the substrate with a mesa width that varies with distance along at least a part of the length of the optical waveguide, forming a waveguide core on the mesa structure by depositing a spin-coatable material over the mesa structure with a thickness of the spin-coatable material at any point along the length of the optical waveguide being defined by the width of the mesa structure at that point, and forming an upper cladding layer above the waveguide core. The step for depositing the spin-coatable material can be performed by rotating the substrate, spinning on the spin-coatable material over the substrate, and removing the spin-coatable material from the substrate beyond the mesa structure. Additional steps can be provided for forming a first silicon oxynitride etch-stop layer over the mesa structure prior to the step for forming the waveguide core, and forming a second silicon oxynitride etch-stop layer over the waveguide core prior to the step for forming the upper cladding layer.
Additional advantages and novel features of the invention will become apparent to those skilled in the art upon examination of the following detailed description thereof when considered in conjunction with the accompanying drawings. The advantages of the invention can be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.